caster problem, getting mixed suggestions on how to correct

[Dutchman]

I was wondering if someone could help me out. I have a 1972 FJ40 that has some steering issues. It has an extended shackle lift on some old springs with 4 degree shims in the spring perches and it follows every little rut in the road.
I took it to an alignment shop and they told me my caster is at -2.95 degrees on the left front and -2.79 degrees on the right front.

I have been researching caster angles here on Mud and it looks like I'm suppose to be at +1 degrees.

I took the cruiser to 4 Wheel parts to see what I can do to fix the wandering problem. They told me that my caster is fine at -3 degrees and to not rotate it to +1. They said rotating it to +1 degree would make the steering worse. I'm confused!

Thinking about my -3 degree caster to me makes sense to rotate my axil back to where it is at +1 degrees. I feel like I'm being pushed into every rut as I'm going down the road, but if I do that I would have to take the existing 4 degree shims off and replace them with 8 degree shims.
6 degrees won't be enough to bring me to +1.
I fear that 8 degree shims may rotate my axil so much that I may run into drive shaft issues. Is this true?

Any insight would be most appreciated!
Dutchman

 

[hellbent40]

do you have a pic of this..??

correct me if i am wrong but positive castor is rotating the arms forward from Zero and negative castor is rotating them back ... Fsm says 1 degree back so rotated to the rear of the vehicle..

as far as the shackle lift.. if change shackles this might help...

 

[CruisinFJ60]

whoever you talked to at 4 wheel parts is an IDIOT!

the less your caster the more it will wander and be squigly down the road and will cause you to have more sensitive steering which in a car for handling in turns can be a good thing depending on other deminsions.

The more positive your caster is the less wandering it will do and the more stable the steering input will be with less twitch to it when input to the wheel is applied, this also in turn will give it more of a laid back feel (like your wanting) but on a car that is built for corners it will make it a little more sloppy and slower to respond.

at a minimum for reasonable handling you need to be at 0* anything less will try to wonder and move with the grooves, this is multiplied by a wider tire grabbing more of the grooves.

 

[CruisinFJ60]

do you have a pic of this..??

correct me if i am wrong but positive castor is rotating the arms forward from Zero and negative castor is rotating them back ... Fsm says 1 degree back so rotated to the rear of the vehicle..

as far as the shackle lift.. if change shackles this might help...

you are incorrect.

rotating the axle back 1* like the FSM says will give you 1* of positive caster not negative.

caster is essentially the forward and rear location of the wheel / tire inside of the wheel well, the further forward (to the front of the truck) is positive caster and the further back the more negative. this is mostly a better descriptor of an IFS vehicle but on the dodge live axle (with coil springs) trucks is pushed the axle forward and rotates it a tiny bit to adjust caster, same for fords.

on these trucks if you rotate the axle forward it will give you more negative caster. that is why when you do a spring over and you angle the pinion up you have to do a cut and turn to correct the caster by rotating the knuckles rearward to give you the proper + caster.

since he has so much negative caster it is from the longer shackles in the front of the truck (stock location) rotating the axle forward and giving negative caster numbers. that said with a 4* shim that he has in there those must be some VERY VERY long shackles up there.

 

[Pin_Head]

you are incorrect.

rotating the axle back 1* like the FSM says will give you 1* of positive caster not negative.

caster is essentially the forward and rear location of the wheel / tire inside of the wheel well, the further forward (to the front of the truck) is positive caster and the further back the more negative.

No. Not really.

The caster angle is the relationship between the knuckle turning axis with respect to the verticle axis. If the top of the knuckle axis is behind (to the rear) the verticle axis (with respect to the front of the truck), the caster angle is positive. Google is your friend.

When you say that "rotating the axle" you need to state what it is rotated with respect to or it is ambiguous and can be taken either way.

 

[CruisinFJ60]

No. Not really.

The caster angle is the relationship between the knuckle turning axis with respect to the verticle axis. If the top of the knuckle axis is behind (to the rear) the verticle axis (with respect to the front of the truck), the caster angle is positive. Google is your friend.

When you say that "rotating the axle" you need to state what it is rotated with respect to or it is ambiguous and can be taken either way.

Instead of trying to twist my words please quote the whole section.

Here is what you left off.

this is mostly a better descriptor of an IFS vehicle but on the dodge live axle (with coil springs) trucks is pushed the axle forward and rotates it a tiny bit to adjust caster, same for fords.

and to be more clear for those trying to twist this as well... the rotation on these vehicles (live axle stated above) is the top of the knuckle to the rear of the vehicle when the axle is pushed forward. because the adjustment is on the lower links and the uppers are non adjustable on a factory suspension.

As someone who has aligned more vehicles than I could ever attempt to guess a number at, I apologize for not googling the proper definition of caster but rather giving a descriptor and trying to describe it in more of a way for people to better understand.

so here is the definition.

"
Caster angle is the angular measure from the vertical of the suspension of a steered wheel in a car or other vehicle, measured in the longitudinal direction. It is to the angle between the pivot line (an imaginary line that runs through the center of the upper ball joint to the center of the lower ball joint) and vertical. Car racers sometimes adjust caster angle to take optimize car performance in particular driving situations.
The pivot points of the steering are angled such that a line drawn through them intersects the road surface slightly ahead of the contact point of the wheel. The purpose of this is to provide a degree of self-centering for the steering - the wheel castors around so as to trail behind the axis of steering. This makes a car easier to drive and improves its straight line stability (tendency to wander). Excessive caster angle will make the steering heavier and less responsive, although, in racing, large caster angles are used to improve camber gain in cornering. Caster angles over 10 degrees with radial tires are common. Power steering is usually necessary to overcome the jacking effect from the high caster angle.
Note that the steering axis (the dotted line in the diagram below) does not have to pass through the center of the wheel, this allows the Caster to be set independently of the mechanical trail, which is the distance between where the steering axis hits the ground, in side view, and the point directly below the axle. The interaction between castor angle and trail is complex, roughly speaking they both aid steering, caster tends to add damping, while trail adds 'feel', and returnability. In the extreme case of the shopping wheel trolley the system is undamped, but stable, as the wheel oscillates around the 'correct' path. The shopping trolley setup has a great deal of trail, but no caster. Complicating this still further is that the lateral forces at the tire do not act at the center of the contact patch, but at a distance behind the nominal contact patch. This distance is called the pneumatic trail and varies with speed, load, steer angle, surface, tire type, tire pressure and time. A good starting point for this is 30 mm behind the nominal contact patch." From Caster - Definition

 

[hellbent40]

sorry if i am incorrect...

thought it was the 90 degrees like it shows in the fsm if rotated toward the rear of the truck then it would be 89 degrees which is -1 degree castor if it is rotated forward to the front of the truck then it is 91 degrees which would be +1 degree castor...

 

[hellbent40]

and yes i do know about a cut and turn... i did mine on my truck and i set the pinion at 10 and the knuckles at 14 which gives me 4

cut and tig welded...

 

[Pin_Head]

Sorry. I didn't mean to touch a nerve. I worked for 3 years doing wheel alignments and I just wanted to clear up the definition of caster. I had no intention of twisting your words. I just felt that what I left off wasn't relevant.

 

[1911]

whoever you talked to at 4 wheel parts is an IDIOT!

Yes, this is pretty much true of anyone and anything there in my experience. 4WP is a great place IF you are a high-school kid who wants your F-150 lifted and blinged-out, for anything else they are complete cretins. As far as Cruisers go, they couldn't find their own ass with both hands. Take it to a real alignment shop.

 

[CruisinFJ60]

sorry if i am incorrect...

thought it was the 90 degrees like it shows in the fsm if rotated toward the rear of the truck then it would be 89 degrees which is -1 degree castor if it is rotated forward to the front of the truck then it is 91 degrees which would be +1 degree castor...

numerically correct yes.. but what it does to the actual steering caster is increases it by +1 as would be shown on an alignment machine if you were to somehow rotate it while it was all bolted together still (which you cant) but you get the idea.

Sorry. I didn't mean to touch a nerve. I worked for 3 years doing wheel alignments and I just wanted to clear up the definition of caster. I had no intention of twisting your words. I just felt that what I left off wasn't relevant.

Didnt mean to get so twisted, my nap did not cure my headache.

 

[FF LC FREAK]

Here's a couple of silly questions. How long are your shackles bolt to bolt? And which way is the fat part of the shim facing - front or back (on the front axle)?

 

[Slow Poke]

I have 2 inch oversize shackles. I have 6 degree shims in front and 4 degree shims in the rear. Tracks beautifully and no drive line vibrations. Put the fat end of the shim towards the shackles. You'll need longer spring centering pins. I used 3/8th inch allen head cap screws.

 

[kling-on]

Yes, this is pretty much true of anyone and anything there in my experience. 4WP is a great place IF you are a high-school kid who wants your F-150 lifted and blinged-out, for anything else they are complete cretins. As far as Cruisers go, they couldn't find their own ass with both hands. Take it to a real alignment shop.

I take offence to that ... not all shops have highschool kids that dont know s*** ... most are very well trained ASE techs ... so dont toss around blacket coments cause you had a bad dealings with someone

here ya go Trent this show what your talkin about better

 

[CruisinFJ60]

I take offence to that ... not all shops have highschool kids that dont know s*** ... most are very well trained ASE techs ... so dont toss around blacket coments cause you had a bad dealings with someone

here ya go Trent this show what your talkin about better

Ah thanks Joel

 

[jp78fj40]

I like that illistration!!!!
does anyone run neg caster?
I'm sorry, I didnt read every post but pos. 1* sounds right for stock. 3-4* pos is what i'm going for when I cut & re-weld my front end.

Adding any length to the shackles would move you to 0 then neg...

 

[numby]

I often think of a shopping cart wheel as an example of negative caster and a bike front end as positive. Not exactly correct but it can occasionally help illustrate the point to someone who's not interested (or capable) of understanding Ackerman angles, etc.

 

[1911]

I take offence to that ... not all shops have highschool kids that dont know s*** ... most are very well trained ASE techs ... so dont toss around blacket coments cause you had a bad dealings with someone

Maybe most around San Antonio; for the ones in north Texas and New Mexico at least, I stand by my comments. Sorry if you find them offensive.

 

[FJ40Jim]

I like that illistration!!!!
does anyone run neg caster?

Only AutoX-ers building a car that will be trailered to events.

On the street, negative caster causes negative stability (the vehicle will not go straight), which is dangerous.

 

[Pin_Head]

I know that this is going to date me as an old man, but 60's and early 70s US land yachts had stock suspensions with negative caster. Maybe that is why they handled like boats.